The present invention relates to a system for detecting a fault such as a shortcircuit in an electric power system, and more particularly to a system for a current compensation type under-voltage detection.
A current compensation type under-voltage detection system having a characteristic shown in FIG. 1 is advantageous in distinguishing between a fault and a normal load condition; i.e., between a condition wherein a fault is present in the electric power system and a condition wherein there is no fault and the electric power system is connected to the load. The current compensation type under-voltage detection system is therefore widely employed as a fault detection system. The characteristic shown in FIG. 1 has been realized by a transistor type relay, as described in Japanese patent application publication No. 21,395/1964 (Japanese Pat. No. 440,087).
Recent development in digital technology has enabled use of a digital arithmetic operation device in a relaying system. In such a digital relaying system, electrical quantities such as currents and voltages in an electric power system are detected at a prescribed interval, and digitized, and are then applied as digital data to a digital operation device. The digital operation device performs arithmetical and logical operations on the applied data, in accordance with a program, to make judgements necessary to operate relaying devices. However, it has hitherto been impossible to realize a relaying system having a characteristic similar to that shown in FIG. 1. The reason for this is that the characteristic shown in FIG. 1 is closely related to the principle of judgement of a transistor type relaying system, which detects the instantaneous maximum value of the electrical quantity, as described in Japanese patent application publication No. 21,395/1964, while in a digital relaying system, the electrical quantities are sampled at certain intervals and are supplied to the digital operation device, thus making it difficult to determine accurately the instantaneous maximum value of the electrical quantity.